Needs, both of environment and economic nature, of processing metal-containing sludges regardless of their origin, are widely known, in order to make such sludges no longer toxic and to recover metals which would be lost with them.
Said sludges, even if they are placed in discharge places that meet the environment necessities, are always a pollution danger of water-bearing strata and a resource waste which resources when exhausted will not be available. As stated above, "even if", because there are not sufficient safe discharge places and not all the sludges finish to a discharge place.
Therefore, the Applicant of this application had already suggested previously a method and device for treating such sludges in the Italian Patent no. 1,232,862, entitled "Metodo e Dispositivo per il Trattamento di Fanghi Contenenti Residui Metallici", (A Method and Device for Treating Metal Residue-Containing Sludges), issued on Apr. 30, 1992 (corresponding application number 67501-A/89 of Jun. 20, 1989).
Briefly, in such patent it was foreseen for metal-containing sludges a washing in equicurrent in a rotating chamber which rotates in a one sense.
However, the yield of such kind of washing was not completely satisfying, or it was not much efficient and sludge contained appreciable traces of metals.